TDR was developed assuming one-dimensional electromagnetic wave propagation in dielectric materials. In one embodiment, it is applied to detect the breaks in cables of coaxial transmission lines, telecommunications, and more recently applied to TV and computer networks. As one example, a voltage is applied to an end of a cable and a discontinuity in the cable causes a voltage reflection. By recording the reflection time, the distance to the discontinuity can be calculated if the apparent dielectric properties of the cable are known. FIG. 3-1 shows a schematic description of TDR system as a multi-section transmission line. It includes a TDR apparatus with step voltage generator and data acquisition system, a coaxial cable, and a measurement probe (non-insulated) embedded in a soil. The measurement probe includes a probe head and a plurality of spaced apart TDR probes or “spikes.” The apparent dielectric constant, Ka, can be determined by the travel time analysis of the TDR waveform for a known probe or spike length.
The current used Purdue One-step TDR method provides a procedure for measuring the water content w and dry density ρd of soil in the field, using the TDR device, with use of soil-specific calibration coefficients obtained from laboratory TDR tests. The method utilizes Ka and the bulk electrical conductivity of soil, ECb, for predicting w and ρd, where ECb for a given probe configuration can be obtained from measuring the waveform reflections and characteristic impedance of a cable. The Purdue One-step TDR method can be used with a range of water contents, especially if assisted with modeling between density-normalized ECb and w. However, the method may be sensitive to variation in compaction energies.